Oxygen range

[tartare]

Recently saw the phrase `oxygen range' referred to under range circles, in the context of a divert.
It's an expression I've never heard of before.
I'd assumed it means the range circle within which stored oxygen will last in the event of a pressurisation failure, allowing one to continue to fly at altitude - but that can't be right - isn't SOP to descent to a safe altitude to avoid hypoxia?
Puzzled...

[wiggy]

I'd assumed it means the range circle within which stored oxygen will last in the event of a pressurisation failure, allowing one to continue to fly at altitude - - but that can't be right - isn't SOP to descent to a safe altitude to avoid hypoxia?


Chart/circle specifics will depend on the operator but they are quite possibly calculated on the basis that do that you do descend.

Certainly for ETOPS purposes our range circles, equal time points and fuel planning are calculated (amongst other things) on the assumption that for a decompression we will perform an immediate descent from the cruise to much a lower altitude as detailed in the QRH, typically FL140/FL150.

As an example if we're over the Atlantic and inside the ETOPS "circle" for say Keflavik (and we always have to inside a circle for somewhere) in the event of a decompression we know we can immediately descend to whatever the altitude the checklist requires, and then fly onwards to KEF and arrive there with at least legal fuel reserves. If we're well inside the circle when things go pop we may well be able to decend to an even more comfortable level enroute to the diversion destination.

[Tinstaafl]

Depends on what rules you have to use. Under FAR135 (the ones I'm most familiar with right now), up to 15,000' cabin altitude you have to have enough 02 for 10% of the pax (rounded up to the nearest whole number), + crew for the entire time for flight between A120 & A15 (also crew O2 for any flight >30mins between A100 & A120).

Allowing for all occupants to use O2 during the descent, crew to use 02 for the entire divert time until below A100, and the 10% pax bit, there will be a maximum range you can fly at A150 for the available 02.

02 may or may not be the most limiting range. You also have to consider other possibilities eg 1 inop fuel needed with driftdown to 1 inop altitude, and also the fuel needed at the O2 altitude.

Or how about gear extended range? If you can't get the wheels to retract then you'll be limited to a lower altitude (and speed!), and probably also have a higher fuel burn at that speed compared to cruise.

They're all independent, but one of them will be the most limiting.

[wiggy]

Yep, we are non FARS but the rules sound similar, i.e. only a percentage of the passengers assumed to remain on O2 once the initial descent is complete, but all crew until at or below 10k.

We work on the worse case of engine out only, depress only and engine out combined with depressurisation to come up with ETPS and "range circles". Since we don't use gear for rapid descents that option isn't planned.


Rgds.

[B737900er]

Worst case would be a decompression to FL100 with both engines running surely?